The invention relates to a method for electrochemically processing a workpiece by means of a pulsating or alternating electrical voltage. A particular area of application for the invention is the removal or deposition of material on a workpiece surface to produce fine structures in the micrometer or sub-micrometer range.
It is possible to produce structures in the xcexcm range by means of mechanical cutting methods such as turning, boring and milling using suitable tools, such as for example diamond cutting tools. In the case of hard alloys and sinter metals where it is not possible to use the metal-cutting deformation method it is possible to achieve dimensional accuracy of a few xcexcm by spark erosion and laser processing.
The smallest structures of some nm in size and dimensional accuracy are achieved using photo-lithographic techniques which are preferably used to produce semi-conductor chips and also to form synthetic material moulds and silicon moulds for micro-teeth wheels and motors. However, the corresponding structures which are obtained by means of anisotropic etching are always dependent upon the crystallographic directions of the substrate.
Electrochemical methods for material processing are known in various embodiments but owing to their limited precision they have hitherto only been used to a small extent. In the case of conventional methods a direct voltage is applied between two electrodes which are immersed in an electrolyte and of which one forms the workpiece and the other the tool. The local current density and thus also the removal and deposition rate are only slightly influenced by the shape of the electrodes. For this reason, the electrochemical reaction also always occurs in electrode regions which are greatly distanced from the actual site to be processed. This sets limits for the achievable accuracy or spatial resolution of the structure produced.
In the case of conventional electrochemical boring, the cathode is lowered into the workpiece and a working gap of 0.05 to 2 mm is formed depending upon the feed rate and current density. The precision which can be achieved is determined in this case by the current density and is therefore relatively low; the smallest achievable edge radius is approx. 0.1 mm. The same also applies for the known electrochemical turning/boring process with the aid of a capillary which is filled with electrolyte and which is used in place of an electrode which is moved against the workpiece in order to direct the reaction current locally to the site to be processed.
An electrochemical processing method which is described as xe2x80x9cScanning Electrochemical Microscopyxe2x80x9d and is intended to render it possible to produce the smallest two-dimensional structures of a few 100 nm in size was presented by A. J. Bard inter alia (cf. Electroanal.Chem. 18 (1994)). In this method, a laterally insulated ultramicro-electrode is guided very closely over the surface to be modified. Preferably by applying a direct voltage reagents are produced at the micro-electrode, the said reagents diffuse to the surface of the workpiece and modify said surface. The minimum size of the structure is determined by the diffusion length of the reatands.
An electrochemical material processing which produces structures in the nm range by means of extremely short voltage pulses was described by R. Schuster inter alia in Phys.Rev.Lett. 80, 5599-5602 (1998). In the case of the tests reported therein short voltage pulses were applied at a duration of xe2x89xa6100 ns and an amplitude up to 4 V between tip and probe of an electrochemical scanning tunnel microscope. It was possible in this manner to produce holes of approx. 5-10 nm diameter and approx. 3 monolayers deep on a gold surface. The reverse process of depositing Cu clusters by the reduction of ions from the electrolyte was possible. Care was taken in these experiments that the furthermost front part of the tip was extremely close to the surface, at a distance of only 1 nm. Owing to such a small distance, large areas of the electrochemical double layer were charged at the tip even during the first 10xe2x88x9210 seconds of the voltage pulse. The fact that the structures produced were nonetheless smaller than these areas is explained by the fact that during charge reversal of the double layer practically all ions in the extremely narrow space are consumed. This leads to the almost complete depletion of the electrolyte in the narrow gap, extending over several 100 nm2, between the tip and the probe. The short pulse duration is not sufficient to replenish the electrolyte in the gap by means of lateral diffusion from the adjacent electrolyte volume. The electrolyte resistance in the gap is therefore infinitely large; the electrode surfaces are mutually xe2x80x9cisolatedxe2x80x9d. Only at the furthermost front end of the tip, where only approx. 3 removing agent molecules are still located between the tip and probe, do the double layers contact each other without there being pratically any electrolyte therebetween and a material-influencing reaction can only occur at this site.
In the aforementioned method the localised removal therefore relates to the local depletion of the electrolyte within a gap which on the one hand must be extremely narrow (1 nm!), on the other hand however in the case of this narrow gap it is necessary to have a predetermined minimum surface expansion. It creates technical problems to achieve this in the case of an industrial application.
All the above discussed electrochemical processing methods, insofar as they are suitable for producing extremely small structures, can only define in a two-dimensional manner the structures formed. A reproducible resolution in three dimensions, i.e. a good relative dimensional accuracy even in the depth direction, using the hitherto known electrochemical methods can at best be achieved in the case of xe2x80x9clargexe2x80x9d structures in the range of some ten xcexcm.
The object of the present invention is to provide a method for electrochemically processing material, which method is suitable for forming even the smallest structures down to the sub-micrometer range and with which method it is possible to achieve a good dimensional accuracy in all three spatial dimensions.
It follows from this, that in the case of the method in accordance with the invention the resolution-determining site selection of the processing relates to local limitation of the charge reversal of the electrochemical double layer. This is a principal difference to the method known from the above mentioned publication, wherein the localised removal relates only to depletion of the electrolyte and to the excessively slow lateral diffusion of ions in the extremely narrow space between the tip (tool electrode) and probe (workpiece).
The local limitation of the double layer charge reversal is achieved in accordance with the invention by virtue of the predetermined dimensioning of the mean value of an applied pulsating or alternating electrical voltage and the voltage deflections measured relative to this mean value with respect to their duration and amplitude in order to define a distance range within which a double layer charge reversal occurs on the workpiece which is sufficient to bring about the desired electrochemical reaction, whereas workpiece areas situated at further distances do not experience a sufficient double layer charge reversal. The shape of the space between the workpiece and the tool electrode is proportioned in such a way that only points of the area of the workpiece to be processed lie within the said distance range.
The applied voltage can consist of pulses of any shape over a rest level or an alternating voltage of alternating polarity with or without direct current components, wherein the wave form of the alternating components can also be sinusoidal. In preference, a periodic voltage curve is to be selected.